A switching regulator is constructed to generate a regulated output voltage by repetition of a switching operation of a switching device, and therefore the switching device generates unnecessary radiation noises at the time of switching operation. The regulated output voltage does not vary so much in the stationary condition in which the operating condition of a load to which the regulated output voltage is applied from the switching regulator is substantially unchanged.
As a result, the switching device performs switching in a uniform pattern. The noise level largely increases in this condition, because the noise components concentrate on the frequencies, which are integer times of the switching frequency of the switching device.
To decrease the noise level, in JP 2003-153526A, the switching frequency is randomly varied thereby to avoid concentration of the switching noise frequencies on the specific frequencies. This method, however, is directed to a pulse-width modulation type switching regulator, and hence the frequency of carrier waves applied to a pulse-width modulation circuit. This method is therefore not useful for a frequency modulation type switching regulator.
The frequency modulation type switching regulator varies the switching frequency in accordance with load conditions. Therefore the frequency modulation type is more advantageous to reduce noises than the pulse-width modulation type. The frequency modulation type still needs to reduce noise generation, because the noise components concentrate on the specific frequencies in the stationary condition.